This project has two approaches, one investigating the bidirectional communication between parasympathetic nerve development and SMG epithelial morphogenesis, and the second identifying embryonic mouse salivary gland stem/progenitor cell populations within the gland. The maintenance of a progenitor cell population as a reservoir of undifferentiated cells is required for organ development and regeneration. Parasympathetic nerves are also a crucial component of the progenitor cell niche during development as they maintain a pool of K5+ progenitors for organogenesis. Injured adult organs do not regenerate after parasympathectomy, and there are few treatments to improve organ regeneration, particularly after damage by therapeutic irradiation. We are investigating the function of neurotrophic factors produced by the salivary epithelium and whether they can restore parasympathetic function after gland damage. We used SMG culture containing fluorescently labeled progenitors (K5-venus), and injured the tissue with irradiation. The epithelial progenitors survived, parasympathetic function was diminished, and epithelial apoptosis reduced expression of the neurotrophic factors. Furthermore we examined biopsy tissue from adult human salivary glands damaged by therapeutic irradiation that also had reduced levels of neurotrophic factors, although nerves were present. We will continue to investigate the effects of irradiation on salivary glands in vivo and the function of surviving nerves in gland pathology. The cell surface receptor Kit has been used to isolate stem/progenitor cells from adult SMGs. These Kit+ progenitor/stem cells can be transplanted into irradiated adult SMGs to regenerate the damaged tissue and restore secretory function. The mechanisms of Kit+ function and Kit+ progenitor cell-mediated regeneration are not yet understood. We are investigating how Kit signaling influences progenitor cell maintenance and expansion. It is not clear which signaling mechanisms coordinate progenitor expansion with establishing organ architecture. We have discovered an interaction between Kit and Fgfr2b that regulates epithelial organogenesis. During development, the SMG end buds express Kit, and Fgfr2b is expressed throughout the epithelium. The Kit pathway increases Fgfr2b signaling. Combined Kit and Fgfr2b signaling regulates transcription factors, including Etv4 and Etv5, resulting in proliferation of the Kit+ cells in the endbud. We have analyzed SMGs from mice with mutations in the Kit receptor. The SMGs are smaller and have reduced number of Kit+ progenitor cells. Thus, Kit and Fgfr2b are essential for progenitor expansion during epithelial organogenesis. Understanding the cell lineage of progenitor cells within the salivary glands will be important from the clinical perspective where progenitor cells of specific lineages may be more appropriate than pluripotent stem cells for clinical transplantation to regenerate irradiation-damaged salivary glands.